Among those things that never happened but everyone believes did - up there with Marianne Faithfull and the Mars Bar - is silent screen star Fatty Arbuckle's assault with a bottle on actress Virginia Rappe in a San Francisco hotel room, as a result of which encounter she was said to have died of internal injuries.

Stahl's novel reclaims Arbuckle through a double retrieval: arguing him to be the scapegoat of the studio people to appease the Christian lobby; and giving him the voice he never had as a silent star. Rather as James Ellroy has taken to ventriloquising recent US history, Stahl exploits similar tabloid material to perform a legitimate exposé. With such a fabricated town as Hollywood, this is a useful service even if Stahl never quite matches the loaded authenticity of his introduction set in the 1980s with him face down on the lawn of a house that once belonged to Arbuckle.

Stahl had been scoring a potent, short-lived combination of Doredin and Codeine 4, known as Dors'n'fours, which offered "a slow-motion rush that lasted half an hour, with a residual opiate buzz that kept you scratching your nose and not moving your bowels for days at a time". The point being that Hollywood has always been a drugs town, serviced in Arbuckle's day by Captain Spaulding, "Drug Dispenser to the Stars", whose arrest cost the studios half a million in hush money.

For his subject, Steven Carter opts for a double impersonation: the mercurial Howard Hughes and his leaden-footed biographer, author of Melville and the Whale, a surprise best seller, resulting in lucrative Rolling Stone commissions involving Madonna and her option of the book. Again, nothing matches Carter's opening, a sly parody of the tedious thanks many American authors feel obliged to offer ("Hannah, I'll never eat lasagne again unless it's yours"), making writing seem more like a larky cooperative venture than solitary endeavour.

I, Fatty owes debts to Sunset Boulevard and Kenneth Anger's Hollywood Babylon, and is a case of successful mimicry that relies on skills of impersonation more usually associated with acting than writing. Risking more, I Was Howard Hughes pancakes more often in its so-so contrasts between Hughes's imaginative daring as mogul, inventor and movie tycoon, and his dull biographer whose sole purpose is to miss the point. The device soon palls, as Carter proves such a sharp impersonator of Hughes in his diaries. The courtship of Katherine Hepburn works in its own right as a comedy of manners, full of well-timed asides: of Garbo, "Greta's great but she's a clinger".

Both writers claim their subjects for fiction on the grounds that their lives were distortions of the truth, given the nature of Hollywood and celebrity, and an ingrained madness running like a fault line through the entertainment system, of which Arbuckle was an early victim and Hughes an exemplar. Hughes walked into Hollywood at the top, thanks to inherited wealth and an air of casual formality (sneakers with a dinner suit) that beguiled many, including Hepburn. Arbuckle worked his ticket the hard way, after a dirt-poor childhood at the mercy of a drunken, bully of a father (a fine portrait of domestic tyranny). Vaudeville led to a career in the movies as a Keystone Cop for the foul-breathed Mack Sennett, then a star and director in his own right, more popular than Chaplin and the first screen actor to turn a million bucks a year.

Arbuckle was an alcoholic and a drug addict, thanks to medical mismanagement, stood five seven and weighed nearly 300 lbs and seems to have been impotent most of his life. Unlike other celebrities who blew up - Orson, Elvis, Marlon - he was fat from the start, and notes forlornly that Chaplin wasn't a tramp when the cameras stopped rolling (he could have made a world-class accountant) but "a Fatty stays fat".

Arbuckle's career occurred within the short space it took for Los Angeles to be transformed from a frontier "cow town packed with transplanted White Trash, first-generation Euro-escape artists, marginal theatrical types, and native Mexicans" to a company town. Arbuckle's framing - he was innocent - and trial were the first major steps in the big clean-up that led to the appointment of Will Hays, an ex-postmaster general, to run a censorship board to appease Washington. Hays didn't come cheap, at a salary of $150,000, two million in life insurance, a limitless expense account and complete discretion over what was moral and what wasn't.

The clearest demonstration of the genius of the system comes after Arbuckle's acquittal - despite which he can't get hired except working as a director under a pseudonym - when he is taken on by William Randolph Hearst, whose newspapers had tried hardest to destroy him. Puzzled as to why he is now being asked to direct Hearst's inamorata Marion Davies on the quiet, Arbuckle is told that he sold more papers for Hearst than the sinking of the Lusitania: "Least I can do is offer you a little job."

Apart from its obvious virtues as biographical writing and recreating a lost era, Stahl's book makes an important point in identifying hysteria as a recurring driving force in the American psyche.